1. Field of the Invention
The present invention relates to a polytetrafluoroethylene (PTFE) containing coating composition and the use thereof. More particularly, it relates to a coating composition containing PTFE having a molecular weight of at least 5,000,000 and an article having a surface which is coated with said PTFE such as a fixing heater roller and a cooking utensil.
2. Description of the Prior Arts
To impart non-tackiness and stain-proofing properties to various articles, a fluororesin, particularly PTFE is coated on the surface of the articles.
Since PTFE has very poor adhesivity to the article material, various attempts have been made to improve the adhesivity of PTFE to the material by adding additives to the PTFE containing coating composition. However, none of the conventional PTFE containing coating composition is satisfactory.
Particularly, a cooking utensil is used under very severe conditions such as a very high temperature and has to withstand rubbings with a metallic or wooden tool or corrosion by seasonings such as salt, vinegar, oil, etc., for an extended period of time.
In an electrostatic process copying machine which is now mainly used as a copying machine or a facsimile machine, a toner image is fixed onto a sheet of paper by heating and pressurizing the sheet of paper bearing the toner image between a fixing heater roller and a pressure roller.
During the fixing of the toner image on the paper, the peripheral surface of the fixing heater roller is heated at a temperature of 150.degree. to 200.degree. C. so that the toner image to be fixed is melted onto the surface of the paper. Therefore, the toner image tends to adhere to the peripheral surface of the fixing heater roller. Once the toner adheres to the surface of the fixing heater roller, an offset phenomenon of the image occurs, and causes the copied image and/or the paper to be dirty.
To prevent the offset phenomenon, the peripheral surface of the fixing heater roller is covered with a fluorine-containing resin having good releasing properties and heat resistance such as PTFE (cf. U.S. Pat. No. 4,568,275). Since PTFE is negatively charged, it easily electrostatically couples with the toner which is generally positively charged so that the toner easily adheres to the PTFE. Further, since conventionally used PTFE has a molecular weight of not greater than about 4,000,000, it is crystalline and has a band structure. Thus, a coating comprising conventional PTFE, tends to have irregularity on its surface and has poor releasing properties.
Also proposed is a fixing heater roller having a peripheral surface which is coated with a tetrafluoroethylene/perfluoroalkyl vinyl ether copolymer (hereinafter referred to as "PFA") and practically used. However, such a fixing heater roller covered with PFA, does not have satisfactory durability or offset resistance.